The present invention relates to a durable electrostatic printing plate including a substrate coated with an image receiving layer and, more particularly, to such a printing plate wherein the image receiving layer includes both amorphous and polycrystalline regions. The invention also relates to a method of making such a printing plate.
Electrostatic printing, sometimes referred to as Xeroprinting, typically utilizes an electrostatic printing plate or roll including a grounded conductive substrate with a permanent (persistent or fixed) image or pattern of insulating material formed thereon. A common method of forming the permanent image or pattern on the surface of the conductive substrate is to deposit a photosensitive polymer layer, also referred to as a photopolymer layer, on the surface of the conductive substrate, such as disclosed in U.S. Pat. No. 4,732,831 to Riesenfeld, et al. Such layer is typically 5 to 50xcexc thick. The photopolymer is exposed to actinic radiation in a desired image or pattern causing the photopolymer to selectively increase its resistivity, producing a persistent image on the electrostatic printing plate. Thereafter, the electrostatic printing plate is charged using corona discharge, causing the latent, high resistivity, insulating areas to build a static charge, while areas of low resistance discharge comparatively quickly. The image is developed for transfer to another surface by toning with oppositely charged particles of toner, in liquid or dry form. The toner is then transferred by electrostatic or other means to another surface such as paper, polymeric film or phenolic resin. Since the original image is fixed in the photopolymer layer, multiple copies can be made with a single exposure of the photopolymer by merely repeating the corona charging, toning and transfer steps.
A mask or photo-tool may be utilized to expose the electrostatic printing plate to light. When the photopolymer layer of the printing plate is exposed to actinic radiation through the mask or photo-tool, the polymeric molecules of the photopolymer become cross-linked in the pattern exposed and an image or pattern is developed in the photopolymer. When the electrostatic printing plate is charged with a corona unit, of the type known in the art, the cross-linked regions of photopolymer retain a high level of electrostatic charge, but the unexposed, uncross-linked regions quickly dissipate the charge. Alternatively, a photopolymer may be selected that reduces cross-linking when exposed to actinic radiation, which likewise produces a persistent image or pattern of contrasting high-resistivity and low-resistivity regions on the surface of the electrostatic printing plate.
A problem with existing electrostatic printing plates is that while the substrate component of each of the plates may be comprised of a durable metal material, the photopolymeric coating is comparatively soft and is subject to damage, thereby negatively effecting the useful life of the plate. In recognition of the foregoing, a method of protecting the photopolymeric coating was developed. Such method is disclosed in U.S. Pat. No. 5,011,758 to Detig. This method involves protecting the photopolymeric layer with a polymeric film. However, the disclosed, protective polymeric film also has a limited useful life, especially when abrasive toner materials are utilized such as glass, metal or inorganic powders like oxides and sulfides.
Photoreceptor plates are frequently used for the transfer of toner images to a receiving surface. Amorphous silicon photoreceptor plates are one of the commercially useful types of photoreceptor plates utilized in standard, toner-based copy machines and laser printers. Photoreceptor plates of amorphous selenium, amorphous selenium alloys of arsenic and tellurium, and organic photo conductor (OPC) plates are also commercially used. These amorphous materials have a much longer useful life than the photopolymer layer or the protective polymer films, providing for millions of copies from a single photoreceptor plate. A typical amorphous photoreceptor plate is 25 to 50 micrometers thick on a rigid aluminum plate or drum. The amorphous structure of such photoreceptor plates allows electrostatic charge to be retained for useful periods of time (as little as a few seconds). However, if the material of the plates crystallizes, the inter-crystalline grain boundaries create regions of high electrical conductivity resulting in the immediate discharging of the plate. This phenomenon has been exemplified in early amorphous selenium plates that were subjected to unexpected crystallization caused by thermal cycling and trace metal contamination in the air. This catalytic crystallization caused these selenium plates to fail since they were unable to store electrostatic charge for sufficient periods of time. The literature teaches away from exposing receptor plates to trace elements that could lead to catalytic crystallization of the amorphous layer of the receptor plate.
However, in a non-analogous field, fabrication of polycrystalline silicon thin film transistors, an amorphous film of silicon was selectively crystallized in areas directly in contact with a toner containing trace impurities. U.S. Pat. No. 5,275,851 to Fonash et al. describes a process that selectively changes the state of an amorphous silicon thin film from amorphous to polycrystalline by depositing trace quantities of palladium in a desired image or pattern in contact with the amorphous silicon thin film. In such process, the silicon layer is heated to a temperature of approximately about 600xc2x0 C., which nucleates the catalytic crystallization of the amorphous silicon in contact with the palladium, but does not nucleate the remaining amorphous silicon layer. The heating of the silicon layer selectively crystallizes the initially, completely amorphous layer of silicon thereby creating a desired image or pattern for polycrystalline thin film transistors. See, also, U.S. Pat. No. 5,147,826 to Liu et al., which describes converting amorphous silicon to polycrystalline silicon using annealing temperatures in the range from about 550xc2x0 C. to 650xc2x0 C. after depositing nucleating sites on the surface of the amorphous silicon film.
It is an object of the present invention to selectively crystallize a film of an amorphous material, creating a persistent image or pattern on an electrostatic printing plate, which can be used to electrostatically transfer a high-quality and high-contrast image to a receiving surface with less wear and a longer life than existing photopolymer coated electrostatic printing plates.
There is provided an electrostatic printing plate comprising a rigid or flexible substrate coated with an image receiving layer that includes an amorphous region and a polycrystalline region. The image receiving layer is preferably comprised of silicon, selenium or their alloys. The invention also relates to a durable electrostatic printing plate or drum that is fabricated using a process that causes an amorphous, insulating layer to selectively crystallize in a desired pattern, which can then be used to repeatedly transfer dry or liquid toner to a receiving surface.
In one preferred embodiment, an electrostatic plate includes a metal substrate with a silicon layer deposited thereon. A palladium-containing toner is subsequently deposited on the amorphous silicon layer in a desired image. Then, when the amorphous silicone layer is heated to an adequate temperature, the palladium nucleates the catalytic crystallization of the amorphous silicon causing a polycrystalline pattern to develop in the amorphous silicon. The amorphous and polycrystalline silicon layer is highly durable and resistant to wear from subsequent use with abrasive toner particles, allowing millions of images to be transferred from a single electrostatic plate or drum.
Alternatively, the palladium-containing toner could be selectively applied to the metal substrate, and the amorphous silicon layer could subsequently be deposited. In addition, a mask could be used to selectively deposit trace quantities of palladium on the surface of either the substrate or the amorphous silicon layer.
While the time that is allowed for the nucleation of the amorphous silicon may depend on many factors, including, for example, the heating temperature, the type of amorphous, insulating layer, the type of trace impurity used to nucleate catalytic crystallization and concentration of the trace impurity, one of ordinary skill in the art would be able to determine the time necessary for nucleation using a few simple benchmark experiments known in the art.
As a representative embodiment of the present invention, an amorphous silicon film coated with a palladium-containing toner caused nucleation of polycrystalline grains when heated to a temperature of about 550 to about 600 degrees centigrade for 5 to 10 minutes. The polycrystalline nature of the silicon film was verified by the thin films inability to hold an electrostatic charge for more than a few seconds.
The present invention is further directed to a method of fabricating the durable electrostatic printing plate.
Further objects and advantages of the present invention will be apparent to those skilled in the art from the detailed description of the disclosed invention.